Method of cost coating paper

ABSTRACT

COATING BETWEEN TWO CYLINDERS, ONE BEING SMOOTH HEATED TO ABOVE 100*C., AND ENGAGING THE COATING, AND THE OTHER PRESSING AGAINST THE WEB AND BEING SOFT ENOUGH SO THAT MOST OF THE MOISTURE CONTENT OF THE COATING MIGRATES INTO THE OVERDRIED WEB BEFORE THE COATED WEB IS RELEASED FROM THE PRESSURE ZONE.   PAPER AND CARDBOARD ARE CAST COATED IN CONTINUOUS OPERATION BY OVERDRYING THE WEB CARRYING A MOIST COATING UNTIL THE RESIDUAL WATER CONTENT OF THE COATED WEB IS SUBSTANTIALLY LOWER THAN ITS EQUILIBRIUM MOISTURE CONTENT UNDER STANDARD CONDITIONS, MOISTENING ONLY THE OVERDRIED COATING WITH SO LITTLE WATER THAT THE RESULTING COMBINED MOISTURE CONTENT OF THE WEB AND COATING DOES NOT SUBSTANTIALLY EXCEED THE EQUILIBRIUM MOISTURE CONTENT, BUT THE COATING IS SOFTENED, AND PASSING THE WEB AND SOFTENED

T. PLOETZ ETAL METHOD OF COST COATING PAPER April 20, 1971 Filed NOV. 26, 1968 mmEwI m:

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INVILN'I'ORS. THEODOR PLOETZ BRUNO HAGG WOLFGANG BARNSCHEIDT BY W M W AGENTS United States Patent 3,575,707 METHOD OF COST COATING PAPER Theodor Ploetz, Hosel, Bruno Hagg, Monchengladbach, and Wolfgang Barnscheidt, Neuss, Germany, assiguors to Feldmuhle Aktiengesellschaft, Dusseldorf, Germany Filed Nov. 26, 1968, Ser. No. 779,115 Claims priority, application Japan, Nov. 27, 1967, 42/76,072 Int. Cl. B44l 1/44; DZlh .I/22

US. Cl. 117-64 8 Claims ABSTRACT OF THE DKSQLOSURE Paper and cardboard are cast coated in continuous operation by overdrying the web carrying a moist coating until the residual water content of the coated web is substantially lower than its equilibrium moisture content under standard conditions, moistening only the overdried coating with so little water that the resulting combined moisture content of the web and coating does not substantially exceed the equilibrium moisture content, but the coating is softened, and passing the web and softened coating between two cylinders, one being smooth heated to above 100 C., and engaging the coating, and the other pressing against the web and being soft enough so that most of the moisture content of the coating migrates into the overdried Web before the coated web is released from the pressure zone.

BACKGROUND OF THE INVENTION This invention relates to coated papers and other coated, glossy webs of fibrous material, and particularly to the cast coating of paper and related products.

In the conventional cast coating of paper and the like, paper carrying a soft coating composition, whose essential ingredients are a liquid carrier, normally water, an inorganic pigment and a binder for the pigment dispersed in the Water, is pressed with its coated face against a smooth solid surface, and dried. The dried coating duplicates the surface finish of the contacted surface.

Highest gloss is obtained if contact between the coating and the finely polished surface of a steam-heated cylinder or drum is maintained until the coating composition is practically dry so that the continuous Web of coated paper, paperboard, or cardboard spontaneously separates from the drum surface. The apparatus required for performing this known method is relatively costly and is limited to a low rate of production. The drying time required for adequately removing the water from the coating is so long that even a cylinder having a diameter of several meters can only turn slowly if the decomposition temperature of the fibrous base stock is not to be exceeded.

It has therefore been proposed heretofore to predry the moist coating to remove a major portion of the water before the coated web is fed to the smoothing cylinder. In order to make the coating soft enough for the smoothing operation, its surface is moistened again. The necessary amount of Water is relatively small, and the contact time between the cylinder and the coating may be reduced somewhat. However, the cylinder surface is necessarily non-porous and non-absorbent. The water driven out of the coating by the heat of the cylinder surface can be expelled only from tthe opposite, uncoated surface of the web. The rate at which the coating is dried, is thus controlled by the rate of diffusion of water or steam through the coating and the fibrous substrate. It is low enough, particularly with heavy papers, paperboard or cardboard, to make the improvement in output rate over the first-described method unsatisfactory.

It has also been proposed to raise the surface temperature of the smoothing cylinder beyond C. The steam generated at the interface of the cylinder and the coating lifts the coating from the cylinder surface and mars the finish of the coated Web unless the web is held against the surface with a pressure sufiicient to overcome the steam pressure. Even so, the coated material may be defective because of the formation of steam bubbles between the coating and the substrate when the coated material is relieved of pressure, and it has been found necessary to add temperature-activated hardening agents to the coating composition. The last-mentioned method is therefore relatively expensive and limited to coating compositions compatible with the hardening agents.

An object of the invention is the provision of a method for cast coating paper and other webs of fibrous material which is simple, inexpensive and capable of being performed at high output rates on machinery of relatively modest dimensions.

SUMMARY OF THE INVENTION The invention resides in an improvement in the known method of coating a Web of fibrous material briefly described above in which one face of the web is covered with a coating composition essentially consisting of a volatile liquid carrier, such as Water, an inorganic pigment, and a binder for the pigment, the pigment and the binder being dispersed in the carrier. The moist coating so produced is pre-dried to remove a major portion of the water, and an amount of Water sufiicient to soften the dried coating is thereafter applied to the same. The softened coating is held in contact with a smooth, non-adherent surface under sufiicient pressure to smoothen the coating, and the Water is substantially completely removed from the smoothened coating.

It has now been found that the removal of the carrier from the smoothened coating can be performed in a fraction of the time required heretofore if the pre-drying step of the moist coating is controlled in such a manner that the web and the coating are overdried until the residual water content of the coated web is substantially lower than the equilibrium moisture content of the coated web under the usual standard conditions of 73 F. and 50 per cent relative humidity and the coating is hardened.

The amount of water used thereafter for softening the coating is not substantially greater than the amount of applied water required to make the combined moisture content of the web and of the coating equal to the equilibrium moisture content. The amount of water can be held to a minimum when the water is applied only to the coating. and moistening of the overdried web is prevented.

It is then quite safe to remove the moisture from the smoothened coating by contact with a cylinder surface heated to more than 100 C., the upper limit of the temperature being determined by the decomposition temperature of the coating and more usually that of the fibrous material. Because that material is still overdried, it is capable of absorbing the moisture removed from the coating while the softened coating is held in contact with the heated surface. The ultimate drying period is no longer determined by the rate of diffusion of the water through the entire coated web, and by its rate of release from the uncoated web face. Very little water, if any, is evaporated from the uncoated surface in the method of the invention.

It therefore becomes practical to use a highly polished and chromium-plated heated rotating cylinder cooperating with a rubber-faced pressure cylinder under a pressure sufficient to make a circumferential portion of the relatively soft rubber facing concavely arcuate for conforming engagement with the Web while the coating conformingly engages the surface of the heated cylinder. When the diameter of the pressure cylinder is at least one-third of the diameter of the heated cylinder, the circumferential length of the pressure zone along the concave rubber face becomes sufficient, even at high, rotary speeds, that much of the desired redistribution of the moisture in the coated web can take place in the pressure zone and at least half the moisture of the coating is absorbed by the fibrous material of the web before the web leaves the pressure zone. The indicated diameter ratio also prevents deformation of the web (creping).

Other features, additional objects and many of the attendant advantages of this invention will readily be appreciated as the same becomes better understood from the following detailed description when considered with the accompanying drawing.

trates a paper coating plant used for performing the method of this invention.

DESCRIPTION OF A PREFERRED EMBODIMENT The plant illustrated in the drawing, as far as required for an understanding of the method, consists of pieces of equipment known in this art. A web 2 of uncoated cardboard is drawn from a supply reel 4 over guide rollers 6, 8 to a supporting roller 10 on which one face of the web is covered with a coating composition discharged from an airbrush 12. The coated web then travels through a drying zone equipped with two gas-fired infrared heaters 14, 16. The radiation emitted from the first heater 14 is directed toward the uncoated face of the web 2, whereas infrared radiation from the second heater 16 is directed against the coated face. The dried coating then passes under a bank 18 of spray nozzles which apply water to the coating before the coated web enters the nip between two driven cylinders 20, 22.

The unheated cylnder 20 has a rubber facing and the cylinder 22 a highly polished, chromium plated surface. It is heated with steam supplied through non-illustrated hollow trunnions, as is conventional. The coated web 2 is trained over approximately of the circumference of the cylinder 22, and the hot material is festooned over guide rollers in a cooling zone 24 before being wound on a take-up reel 26.

The apparatus briefly described above was operated under the following conditions:

The base stock 2 was cardboard 220 cm. wide having a weight of 220 g. per m. It traveled at a speed of 60 meters per minute. The coating composition consisted of water and 45% solids as follows (in weight percent of total solids):

Kaolin 73.9 Satin white 9.5 Tetrasodium pyrophosphate 0.3 Casein 5.8 Borax 0.5 Acronal 500 9.3 Soap flakes 0.7

Note: Acronal 500 is a commercially available copolymer of acrylic and vinyl polymers (BASF).

The base stock was coated with the above composition at a rate to make the weight of the applied coating solids approximately g./m.

The first infrared heater 14 had an efiective length of 3 meters in the direction of web travel, the second heater 16 a length of 5 meters. The residual moisture in the cardboard was approximately 1 percent after passing the second heater.

The equally overdried coating was uniformly sprayed with water containing O.51% non-ionic wetting agent (a commercial mixture of fatty alcohol polyglycol ethers: Dehypon) at a rate of 8 g./m. The distance between the second heater 16 and the bank 18 of spray nozzles was too short for absorption of significant moisture by the web from the ambient atmosphere, and the nip between the cylinders 20, 22 was so close to the nozzles 18 that there was no significant evaporation of the applied water.

The rubber facing of the pressure cylinder 20 had a hardness of to Shore, and the external diam eter of the cylinder was cm. The polished smoothing cylinder 22 had a diameter of cm., and its cylindrical surface was kept at a temperature of 120 C. by means of superheated steam.

The web 2 covered with a white coating free from defects was spontaneously released from the hot cylinder surface prior to cooling and reeling. The coating surface had a whiteness of 83% as measured with a Zeiss instrument (Elrepho), and the gloss readings obtained on a Zeiss goniophotometer were 118 and 74 based on a barium sulfate reference standard.

It will be appreciated that the specific operating conditions described above are merely illustrative, and those skilled in the art will modify the procedure employed to suit specific materials and available equipment.

More specifically, other conventional coating compositions may replace the one specifically described above without basically affecting the method. While water is the volatile liquid carrier most commonly used in this art, other carriers may be employed. The fibrous material of the base stock will consist mainly of celluose under most conditions of commercial application of the instant method, but the replacement of some or all cellulose by other fibrous materials capable of absorbing water or another suitable carrier for the coating composition is entirely within the scope of this invention. The use of noncellulosic fibers of synthetic organic polymers and of such inorganic fibers as asbestos is specifically contemplated.

The equilibrium moisture of fibrous material suitable for use in the method of the invention generally varies between 2 to 10 percent, and may even be higher, depending on the fiber composition, the higher end of this range being typical of ordinary paper stock. When overdried as described above, the fibrous material will not normally contain more than 1% moisture. Practically all coating compositions now commercially employed for cast coating are soft enough during smoothing at elevated temperature when remoistened with water at a rate of 1 to 12 g., a rate of 4 to 10 g./m. being generally most favor able. A typical heavy paper or cardboard having a total Weight of 200 g./m. and a moisture content of 1% when overdried, has a total moisture content of 5% when remoistened with 8 g. water per square meter prior to smoothing a value well below the equilibrium moisture content.

It has been found that it is not always necessary to prevent moistening of the overdried fibrous material when moistening the overdried coating prior to smoothing, and very adequate results have been obtained with some papers when the coated web was quickly passed through a body of Water for remoistening of the coating. Roller coating, of course, may replace the spraying specifically referred to in the preceding example of a preferred embodiment, and other methods are available to those skilled in the art.

The successful operation of the method requires very careful balancing of operating variables when the moisture content of the coated web is not reduced to 3% or less during overdrying, regardless of the equilibrium moisture of the material, and infrared radiant heaters have been found to provide adequate removal of water from the moist coating and the supporting web at the high rate necessary if the drying zone is not to become excessively long.

The cylinders illustrated and described above are preferred for providing the heat and pressure required in the smoothing operation proper, but continuous belts have been employed in similar applications and may replace the cylinders in apparatus for performing the instant invention.

Experience has shown that good results are obtained over a wide range of web and coating compositions when the pressure applied to the coated web between the cylinders is between 30 and 200 'kg./cm. the optimum pressure differing somewhat with other process variables, but being 40 and 100 kg./cm. under most conditions, and much lower than in most known methods.

The invention does not rely on evaporation of water from the coated web, and the time of contact between the coated material and the heated smoothing drum may thus be very short. The plant more specifically described above operates at a speed which is many times higher than that of known apparatus used in a conventional manner and having cylinders of twice the diameters.

The small amount of steam, if any, generated in the instant method on the smoothing cylinder does not tend to accumulate at the interface of the coating and of the cylinder, and thus need not be overcome by high mechanical pressure. It does not tend to disrupt the bond between the coating and the base stock, and the coating composition thus need not include hardeners, and all conventional cast coating compositions are suitable for the instant method.

The advantages of the invention over the coating methods employed heretofore increase with the thickness of the base stock. They are significant even with the thinnest webs now used as supports for coatings of the type described.

What is claimed is:

1. In a method of coating a web of paper or cardboard in which one face of the web is covered with a coating composition essentially consisting of water, an inorganic pigment, and a hinder, the pigment and the binder being dispersed in the water; the moist coating so produced is dried to remove a major portion of the water; an amount of water sufficient to soften the dried coating is applied to the same; and the softened coating is held in contact with a hot, smooth, non-adherent surface under sufiicient pressure to smoothen the applied coating until the water is substantially completely removed from the smoothened coating; the improvement which comprises:

(a) overdrying said moist coating and the web supporting said moist coating until the residual water content of the coated web is substantially lower than the equilibrium moisture content of said coated web under standard conditions of 73 F. and 50 percent relative humidity, and said coating is hardened;

(b) said amount of water being not substantially greater than the applied water required to make the combined moisture content of the web and of said coating equal to said equilibrium moisture content;

(c) said surface being heated to a temperature between C. and the decomposition temperature of said paper or cardboard; and

(d) said softened coating being held in contact with said heated surface for a period sufiicient that the moisture content of said coating is substantially completely absorbed by said web.

2. In a method as set forth in claim 1, said web being continuous and being coated while moving, the softened coating being held in contact with said surface under said pressure by moving the same between two moving members, one of said members having said surface, the other member having a face more resilient and softer than said surface, the temperature of said face being lower than the temperature of said surface, said face engaging said web.

3. In a method as set forth in claim 2, said pressure being between 30 and 200 kg./cm.

4. In a method as set forth in claim 2, said pressure being between 40 and 100 kg./cm.

5. In a method as set forth in claim 2, said members being cylinders rotating about respective axes, the diameter of said other member being at least one-third of the diameter of said one member, and said face being sufficiently soft to cause a circumferential portion of said face to become concavely arcuate under said pressure for conforming engagement with said web while the coating conformingly engages said surface, the circumferential length of said concave portion being sufficient for holding said web in contact with said portion until at least onehalf of said moisture content of the softened coating is absorbed by said paper or cardboard.

6. In a method as set forth in claim 1, said amount of water being preferentially applied to said overdried coating while moistening of said overdried web is substantially prevented.

7. In a method as set forth in claim 5, said pressure being between 40 and 100 kg./cm.

8. In a method as set forth in claim 1, said residual water content being approximately one percent.

References Cited UNITED STATES PATENTS 2,678,890 5/1954 Leighton 11764 2,759,847 8/1956 1Frost et a1. ll7-64 3,338,734 8/1967 Goff et a1 ll764 WILLIAM D. MARTIN, Primary Examiner W. R. TRENOR, Assistant Examiner US. Cl. X.R. ll7l0, 1S6 

